I've build a bunch of kitesurfboards, a few hydrofoil boards, a few sup now.

Have a blank left and am planning on building a split board for both Hydrofoiling and surf.
I've ordered a set of carbon tubes 26,5mm dia that are used to attach wings to glider fuselage. There's an inner tube that fits perfectly into outer tube. The tube is 1m long so I'll have 500mm for 2 tubes into board. 250mm into each side.

Cool idea. If you will use a foil with a plate, it may ease reinforcement by letting you use a reinforcing plate in he bottom line shinn did.
The embedded tubes technique has been used with success, but could mean introducing a lot of structure/weight/complexity to take the loads from outside into the neutral axis and then back to the skins.
I would probably cut a stepped scarf joint through the board, reinforce its face, and try to carry most loads with plates on the top and bottom.
Challenging project, however you do it.